Groups not being converted to Collections
Background:

In Blender 2.80 both groups and layers have been merged into collections. Since they are technically two different things in Blender 2.79, it made sense to keep them still somehow apart or distinguishable from each other in Blender 2.80. 

Layers are converted to collections, but groups are not going to be converted, since Blender 2.80 does not know, whether you want to create a collection for each group or if you want to use the groups from Blender 2.79 at all. 

The latter is because Blender 2.7x's  group feature wasn't exactly intuitive to use and many people (including me) might have created random groups at one point without having any further use for them. 



Enter Data Block IDs:

The three cubes in this screenshot of Blender 2.79 are in the same group named "cubes". 



If you open the file in Blender 2.80, under "Properties - Object - Collection" you will find a new input/selection box named "Unique Data Block ID name", where you will find the name of your Blender 2.79 group. 



If you click on the arrow to the right of the group name, a drop-down menu opens, where you can select "Select Objects in Collection". Now, all the objects that were assigned to the 2.79 group "cubes" will be selected. You can then assign them to a new collection by hand.